The present invention relates to a hard alloy having excellent hardness, toughness, wear resistance, fracture resistance, plastic deformation resistance and thermal cracking resistance, in which plate-crystalline tungsten carbide (hereinafter abbreviated to "platy WC") is crystallized. Specifically the platy WC is crystallized to a platy WC-containing hard alloy suitable as cutting tools such as an insert, a drill and an end mill, a base material of a coating super hard tool, plastic working tools such as a drawing mold, a die mold and a forging mold and shearing tools such as a punching mold and a slitter. The present invention also relates to a composition for forming platy WC and a process for preparing the platy WC-containing hard alloy.
In general, hardness, i.e., wear resistance and strength and toughness, i.e., fracture resistance of a hard alloy can be changed by WC particle size, Co content and additional amounts of other carbides such that the hard alloy has been widely used for various purposes. However, there is a problem of antimony tendency that if wear resistance is heightened, fracture resistance is lowered, while if fracture resistance is heightened, wear resistance is lowered.
One course for solving this problem includes paying attention to the anisotrophy of mechanical characteristics due to the crystal faces of WC. Specifically, for example, a means relating to a platy WC containing hard alloy or a process for preparing the same, in which platy WC has a shape represented by a triangular plate or a hexagonal plate and has a (001) face preferentially grown in the direction of the (001) face since the (001) face of WC crystal shows the highest hardness, whereas the direction of a (100) face shows the highest elastic modulus, is desirable.
As representative examples of prior art techniques relating to platy WC, there may be mentioned Japanese Patent Publications No. 23049/1972 and No. 23050/1972 and Japanese Provisional Patent Publications No. 34008/1982, No. 47239/1990, No. 51408/1990, No. 138434/1990, No. 274827/1990 and No. 339659/1993.
In Japanese Patent Publications No. 23049/1972 and No. 23050/1972, a process is described for preparing a platy WC-containing hard alloy by using mixed powder which comprises colloidal tungsten carbide powder containing a porous agglomerate for growing platy WC and a powder of Fe, Ni, Co or an alloy thereof.
In Japanese Provisional Patent Publication No. 34008/1982, a process is described for preparing twin tungsten carbide in which (001) faces are bonded as a twin face. This is accomplished by adding a small amount of an iron group metal salt to mixed powder of strongly pulverized W and C and then carbonizing the mixture under heating.
Further, in Japanese Provisional Patent Publications No. 47239/1990 and No. 138434/1990, a process is described for preparing a hard alloy by using, as a starting material, a solid solution of (W,Ti,Ta)C in which tungsten carbide is contained in a super-saturated state and crystallizing platy WC at the time of sintering under heating.
Next, in Japanese Provisional Patent Publication No. 274827/1990, a process is described for preparing an anisotropic hard alloy by subjecting a used hard alloy to oxidation, reduction and then carbonization to obtain powder. The powder is then molded and the resulting molded compact is then subjected to sintering or hot pressing.
In addition, in Japanese Provisional Patent Publication No. 339659/1993, a process is described for preparing a hard alloy containing platy WC by subjecting mixed powder comprising WC with a size of 0.5 .mu.m or less, 3 to 40% by weight of a compound with a cubic structure and 1 to 25% by weight of Co and/or Ni to sintering at 1,450.degree. C. or higher.
In the hard alloys or the hard alloys obtained by the preparation processes described in these 8 publications, the growing rate of the (001) crystal face of WC is low, all of the a axis length, c axis length and c/a ratio of the WC crystal are small and the ratio of platy WC contained therein is low. This results in the problem that all of the various characteristics of the hard alloy, particularly hardness, wear resistance, strength, toughness and fracture resistance cannot be improved. Also, in the preparation processes, it is difficult to control a particle size, and to heighten the ratio of platy WC contained therein. In addition, said processes can be applied only to a hard alloy in which compositional components are limited, and preparation cost is high.